Inequality is Normal: Dominance of the Big Trees

Large-diameter trees in the sugar pine/white fir forest of Yosemite National Park, California, USA (Credit: James Lutz/Utah State University)

The top one percent of the forest has been sharing some vital information with researchers. Utah State University researchers used large forest plots from 21 countries and territories to find that, on average, the largest one percent of trees in mature and older forests comprised 50 percent of forest biomass worldwide. Furthermore, the amount of carbon that forests can sequester depends mostly on the abundance of big trees. The size of the largest trees was found to be even more important to forest biomass than high densities of small and medium trees.

The findings by USU lead author Jim Lutz, assistant professor of wildland resources in the S.J. and Jessie E. Quinny College of Natural Resources, co-author Dan Johnson, a research associate at USU, and co-lead author Tucker Furniss, a doctorate student at USU, were published in the journal Global Ecology and Biogeography.

“Big trees provide functions that cannot be duplicated by small or medium-sized trees,” said Lutz. “They provide unique habitat, strongly influence the forest around them and store large amounts of carbon.”

Lutz and Johnson worked alongside 98 scientists and thousands of field staff as part of the largest study undertaken to date with the Smithsonian Forest Global Earth Observatory (ForestGEO). The findings will have profound implications toward ecological theories and carbon storage in forests. Rather than examining tree species diversity in temperate and tropical ecosystems, the global study emphasized forest structure over a vast scale.

The study shows that the structure of the forest is as important to consider as species diversity - the largest trees follow their own set of rules. Using 48 of the large forest dynamics plots from around the world coordinated by the Smithsonian ForestGEO Program, scientists were able to examine the variability of forest structure on a consistent basis.

“Having a worldwide group of scientists following the same methods offers us unique opportunities to explore forests at a global scale,” Johnson said. “This is a really wonderful group of scientists united by a passion for deepening our understanding of forests.”

Tropical forests typically have many more species than temperate forests. However, the study found that temperate forests have higher structural complexity, both in terms of different tree sizes within an area and also between adjacent areas of forest.

“The distribution of big trees has not been well explained by theory,” said Furniss. “Our results emphasize the importance of considering these rare, but disproportionately important ecosystem elements. We clearly need more applied and theoretical research on these important big trees.”

The researchers also found that the largest trees are representatives of the more common tree species. The ability of some trees in any given forest to reach very large sizes relative to the other trees and concentrate resources seems to be a global phenomenon.

“Big trees are special,” said Lutz. “They take a long time to regrow if they are eliminated from a forest. Making sure that we conserve some big trees in forests can promote and maintain all the benefits that forests provide to us.”

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